Diagnostic tools used for detecting or quantitating biological analytes typically rely on ligand-specific binding between a ligand and a receptor. Ligand/receptor binding pairs used commonly in diagnostics include antigen-antibody, hormone-receptor, drug-receptor, cell surface antigen-lectin, biotin-avidin, substrate/enzyme, and complementary nucleic acid strands. The analyte to be detected may be either member of the binding pair; alternatively, the analyte may be a ligand analog that competes with the ligand for binding to the complement receptor.
A variety of devices for detecting ligand/receptor interactions are known. The most basic of these are purely chemical/enzymatic assays in which the presence or amount of analyte is detected by measuring or quantitating a detectable reaction product, such as gold immunoparticles. Ligand/receptor interactions can also be detected and quantitated by radiolabel assays.
Quantitative binding assays of this type involve two separate components: a reaction substrate, e.g., a solid-phase test strip and a separate reader or detector device, such as a scintillation counter or spectrophotometer. The substrate is generally unsuited to multiple assays, or to miniaturization, for handling multiple analyte assays from a small amount of body-fluid sample.
In biosensor diagnostic devices, by contrast, the assay substrate and detector surface are integrated into a single device. One general type of biosensor employs an electrode surface in combination with current or impedance measuring elements for detecting a change in current or impedance in response to the presence of a ligand-receptor binding event. This type of biosensor is disclosed, for example, in U.S. Pat. No. 5,567,301.
Gravimetric biosensors employ a piezoelectric crystal to generate a surface acoustic wave whose frequency, wavelength and/or resonance state are sensitive to surface mass on the crystal surface. The shift in acoustic wave properties is therefore indicative of a change in surface mass, e.g., due to a ligand-receptor binding event. U.S. Pat. Nos. 5,478,756 and 4,789,804 describe gravimetric biosensors of this type.
Biosensors based on surface plasmon resonance (SPR) effects have also been proposed, for example, in U.S. Pat. Nos. 5,485,277 and 5,492,840. These devices exploit the shift in SPR surface reflection angle that occurs with perturbations, e.g., binding events, at the SPR interface. Finally, a variety of biosensors that utilize changes in optical properties at a biosensor surface are known, e.g., U.S. Pat. No. 5,268,305.
Biosensors have a number of potential advantages over binding assay systems having separate reaction substrates and reader devices. One important advantage is the ability to manufacture small-scale, but highly reproducible, biosensor units using microchip manufacturing methods, as described, for example, in U.S. Pat. Nos. 5,200,051 and 5,212,050.
Another advantage is the potentially large number of different analyte detection regions that can be integrated into a single biosensor unit, allowing sensitive detection of several analytes with a very small amount of body-fluid sample. Both of these advantages can lead to substantial cost-per-test savings.
A key element in the manufacture of biosensors, particularly multi-assay biosensors, is the placement of analyte-specific binding molecules or enzymes at desired locations on a biosensor surface. Ideally, it would be desirable to construct a universal biosensor surface under rigorous microchip manufacturing conditions, but allow a variety of different surface-region formats to be achieved under less restrictive manufacturing conditions, which at one extreme would allow an end user to tailor the universal chip to a unique multi-analyte format.